Over the last century, the human population size and growth rate has exploded. We went from a world where there was never more than 2,000,000,000 humans ever, to now a world where there's nearly 8,000,000,000 people on the planet, and that is rising. In fact, if you're curious, here are some estimates from the UN as to population increases in the coming century. Now it's important to remember that just like there's a carrying capacity in nature, there's a carrying capacity for humans, and there is a limit to global carrying capacity for the human population. Although, estimates for what this is vary. And additionally, that could change based on technological advances. However, the point is we need to be conscious about our ecological footprint, which is the amount of land needed to sustain an organism or a population's use of resources. So if we think about it in terms of an individual there's going to be a certain amount of land that person needs to provide them with the necessary nutrients, right, food and whatnot. Also, that individual is going to generate waste. There needs to be a certain amount of space for that waste to go, so that that person doesn't kill themselves with their own waste. I mean, these are just two examples. But the point is we need to think about our use of this earth, and how sustainable our lifestyles are. Because this is unsustainable. As we saw with our logistic growth model previously, previously, there is a carrying capacity. We just haven't hit it yet. Now I don't mean to fear monger anything, but those are important things to think about. However, moving on, I'm going to talk about demographic transitions. This is maybe getting a little more into sociological territory, but this is a transition from high birth rate and high death rates to low birth rates and low death rates. So, essentially, this is why you see, you know, what are often referred to as developing nations, have many more young people than older people because their birth rates are high and their death rates are high. In more developed nations, we have lower birth and death rates, and so our age pyramids, instead of looking like that, look more like this. Because there's just a higher survival for a large range of angels. Now the fertility rate is the average number of surviving children each woman has in her lifetime. And this factors into what we call the replacement rate, which is the fertility rate required for women to give birth to enough children to sustain the population. And this is something we're thinking about in developed nations now, because as we can see, for example, in places like Japan, their population is decreasing because their replacement rate is, well, not high enough. They're not sustaining their population, their fertility rate is too low. Generally speaking, I should say, replacement rate is about 2.1. Because if you think about it, you have and and this is for developed nations, I should say, this is like US, you know, if you think about it, a woman would have to give birth to 2.1 children. Basically, that's saying that most women need to give birth to 2 children which will replace themselves and, you know, their partner. And, you know, this point one is basically because there's, you know, a chance that not all of these people are going to make it to the age where they can reproduce. You know, they're not going to make it all the way through that generation time. So that's why it's 2.1. But basically, you could almost just think of it as 2 to replace each parent. Alright. With that, let's call it a day. That's all I have for this lesson. See you guys next time.